Mischa Zverev, the brother of Alexander, during his five-set victory over Benoît Paire.CreditElsa/Getty Images

“Today was upsetting,” said Zverev, the highest seed in men’s singles to lose so far. “The way I played was upsetting. The tournament so far is upsetting for me. I know that I could have done some big things here. I know that I could have done something that I haven’t done before. But I won’t.”

With the late withdrawal of second-seeded Andy Murray, Alexander Zverev was the highest seed in his half of the draw, and he had a golden opportunity to make a deep run at a Grand Slam tournament for the first time.

But he still has not made it past the second round at the United States Open. And his best result at a Grand Slam event is reaching the fourth round at Wimbledon this year.

“It’s upsetting because the draw is pretty open in the bottom part,” Zverev said. “I felt like I should have been the favored there. You know, I just played a very, very bad match, so it’s unfortunate. But that’s how it is.”

Both 20, Zverev and Coric have been rivals since their junior days. Coric, a Croat, even defeated Zverev, a German, on his way to the 2013 United States Open boys’ title. They are part of the ATP Tour’s Race to Milan, a points-based competition to earn entry into a year-end tournament featuring the best players younger than 21.

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Fourth-seeded Alexander Zverev in his 3-6, 7-5, 7-6 (1), 7-6 (4) loss to his rival Borna Coric. He is the highest men’s seed to lose.CreditKarsten Moran for The New York Times

Zverev, who has won five titles this year, including two on hardcourts this summer, has an overwhelming lead in the standings, with Coric a distant third. While Zverev had shot up the ATP rankings, Coric has now collected three wins over top-10 players this year.

Coric, in fact, burst onto the tennis scene before Zverev, beating Rafael Nadal as a 17-year-old in 2014. He achieved his career-high ranking, No. 33, in July 2015. But over time he has ceded the spotlight to other next-generation players.

But he did not cede the match to Zverev. In the fourth set, serving at 5-6, Coric fell behind, 0-40, to give Zverev three set points. Coric rallied with two winners, then hit three straight serves that Zverev could not return to get to force a tiebreaker.

Struggling with a neck injury, Coric, ranked No. 61, had won only four matches since reaching the quarterfinals in Madrid in May.

“I had a rough few months,” he said.

Zverev played down the effect his long runs to titles in Washington and Montreal had on his body, and the effect that raised expectations may have had on his mind.

“I’ve been dealing with expectations from a very young age, so for my whole career,” Zverev said. “For me, this is just another step.”

Instead, Mischa, 30, is the last Zverev standing after beating Paire, 6-3, 6-2, 3-6, 6-7 (3), 7-5. In the third round, he will play 10th-seeded John Isner, who defeated Hyeon Chung, 6-3, 6-4, 7-5.

Fifth-seeded Marin Cilic, the 2014 United States Open champion, is now the highest remaining seed in the bottom half of the draw. Cilic, who did not play a tournament between Wimbledon and the United States Open because of injuries, defeated Florian Mayer, 6-3, 6-3, 6-3 on Wednesday night. He faces No. 29 Diego Schwartzman in the third round.

On a night when the tournament lost one rising star, another one grew brighter as 18-year-old Denis Shapovalov of Canada ousted eighth-seeded Jo-Wilfried Tsonga, 6-4, 6-4, 7-6 (3), in a night match at Arthur Ashe Stadium. Shapovalov upset the top-ranked Nadal at the Rogers Cup in Montreal this month during a run to the semifinals. Shapovalov, playing in his first United States Open, will meet Kyle Edmund of Britain in the third round.

Even as much of the focus has been on Nadal and Roger Federer at the Open so far, the tournament has already been a showcase for young players. Frances Tiafoe, 19, pushed Federer to five sets before losing on Tuesday night.

Taylor Fritz, a 19-year-old American, gained his first victory at a Grand Slam tournament, defeating Marcos Baghdatis, 6-4, 6-4, 6-3. He will play sixth-seeded Dominic Thiem, 23, in the second round on Thursday. And Andrey Rublev, 19, will take on seventh-seeded Grigor Dimitrov in another second-round match at Armstrong Stadium on Thursday.

A version of this article appears in print on , on Page B11 of the New York edition with the headline: Only One of Two Zverevs Wins on Day Highlighting Rising Stars. Order Reprints | Today’s Paper | Subscribe